1. Field of the Invention
The invention relates to a demodulation method for a pseudo-heterodyne signal, wherein the pseudo-heterodyne signal has a phase-modulated carrier signal and the pseudo-heterodyne signal is digitally sampled.
2. Description of Related Art
A pseudo-heterodyne signal is designated as a signal emitted from an interferometer that is based on a pseudo-heterodyne method. As is generally known, with an interferometer, two sufficiently temporal coherent—i.e., capable of interference—beams are brought into interference. In principal, the beam of a coherent beam source is split with a beam splitter into a first partial beam and a second partial beam. The paths that the first partial beam and the second partial beam take are called the arms of the interferometer. At the exit of the interferometer, the partial beams are brought together and brought into interference (or beams are brought into interference that are derived from partial beams—depending on the construction of the interferometer). The intensity of the beam at the exit of the interferometer is proportional to the cosine of the phase difference between both interfering partial beams. Changes in the phase difference, e.g., caused by minimal changes in the length of an interferometer arm, thus lead to a change in the intensity at the exit of the interferometer.
In the pseudo-heterodyne method described in the introduction, the phase difference between both partial beams interfering in the interferometer is modulated with a periodic sawtooth signal. Generally, other types of signals are possible, but the use of a sawtooth signal is the best-known and most widely used method, so that the description in the following is directed toward the use of a sawtooth signal, however, the invention is not limited thereto. A signal z(t) is denoted as a sawtooth signal which can be described using the following Fourier series:
                              z          ⁡                      (            t            )                          =                  a          ⁢                                    ∑                              k                =                1                            ∞                        ⁢                                          sin                ⁡                                  (                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                                          kf                      0                                        ⁢                    t                                    )                                            k                                                          Equation        ⁢                                  ⁢        1            
Here, a is a scaling factor that is not zero, f0 is the frequency of the periodic sawtooth signal and t is time. As can be seen from the Fourier series of the sawtooth signal z(t), frequencies occur that correspond to an integer multiple of the frequency f0. The phase difference signal, which arises from the modulation of the, assumed to be constant, phase difference with the known sawtooth signal z(t), is superimposed with the actual, wanted signal s(t) of interest, which basically exists in the phase change of a partial beam in an arm of the interferometer—caused in any manner. The intensity signal at the exit of the interferometer is now the pseudo-heterodyne signal. As the wanted signal vanishes, the pseudo-heterodyne signal is transferred in the non-modulated carrier signal, which is essentially made up of periodical repetitions of sine-shaped sections. The frequency of the periodical repetition of the repetitive sine-shaped sections is denoted as the frequency of the carrier signal here. This frequency corresponds to the frequency f0 of the periodic sawtooth signal z(t). If a wanted signal is present, the carrier signal is phase modulated by the wanted signal.
Mach Zehnder interferometers are a type of interferometer that is often used. Other types of interferometers, for example, Michelson interferometers, are also basically possible to use. Semiconductor lasers are particularly suitable for creating the beam used in the interferometer, such as, e.g., a VCSEL (Vertical-Cavity Surface-Emitting Laser). The advantage of semiconductor lasers is that modulation of the phase difference between both interfering partial beams in the interferometer can be particularly easily implemented. Here, the characteristic of semiconductors is used that the wavelength of the laser beam emitted from the semiconductor laser is dependent on the pumping current. In this manner, the wavelength of the laser beam can be modulated with the desired sawtooth signal z(t) by amplitude modulation of the pumping current with a corresponding signal. The change of the phase difference dφ between both interfering partial beams in the interferometer is linearly related to the change of the frequency of the laser beam dv at a present path length difference l of both partial beams:
                              d          ⁢                                          ⁢          ϕ                =                                            2              ⁢              π              ⁢                                                          ⁢              nl                        c                    ⁢          dv                                    Equation        ⁢                                  ⁢        2            
Here, n is the average refractive index along the path of the path length difference 1 and c denotes the vacuum speed of light. Thus, the result is that the phase difference between both interfering partial beams in the interferometer can be modulated onto the desired sawtooth signal z(t) by corresponding modulation of the pumping current of the semiconductor laser. This modulation method only works for non-vanishing path length differences l of both partial beams. The advantage of this modulation method is that a movable and susceptible component is not needed for modulation of the phase difference between both interfering partial beams in the interferometer, rather only electronic components are used.
The pseudo-heterodyne signal, which is initially present as an intensity signal at the exit of the interferometer, is normally transformed into an electric signal, for example, by a photo diode, in particular a PIN photo diode. Then, after possible amplification, this electric signal is digitally sampled. Digital sampling is carried out in particular by an analog-digital converter. A possible planned amplification can also occur directly by the PIN photo diode.
The above-described interferometric method for creating a pseudo-heterodyne signal is, for example, suitable for such applications in which small deflections are detected, wherein the interferometer is used in such a manner that the deflection to be detected leads to a path length change of an interferometer arm; as a result deflections in a (sub-)wavelength range of the beam source used can be easily detected. The method is thus used insofar, for example, in the field of vibration measurement, in particular, in vibration measuring devices or in measuring devices that detect mechanical oscillation, wherein the type or change of the detected oscillation can lead to conclusions about other interesting factors. Vortex flow measurement, for example, is possible as such an application. In the following, this application will be described as an example. This, of course, does not limit the method according to the invention, but rather a variety of other application possibilities are possible, e.g., application in acoustic sensors.
The volume flow of gases, vapors and liquids in piping systems can be measured with a vortex flowmeter. The measuring principal is based on the principal of the Kármán vortex street. In the measuring tube, an obstruction is found that the medium flows around and behind which a vortex is shed. The frequency f of the vortex shedding is proportional to the flow velocity v of the medium. The dimensionless Strouhal number S describes the relation between vortex frequency f, width b of the obstruction and the average flow velocity v of the medium:
                    f        =                              S            ·            v                    b                                    Equation        ⁢                                  ⁢        3            
The vortexes spreading behind the obstruction in the direction of flow act upon a probe found behind the obstruction in the direction of flow, such as, e.g., a membrane or a rod-shaped probe. The probe is periodically deflected by the vortex with the frequency f, which represents the wanted signal. This deflection can, for example, be transferred mechanically to a mirror of an interferometer arm, it can also be directly detected by an optical fiber, which is attached to the deflectable membrane and is subject to a length change corresponding to the deflection. The deflection detected in this manner is responsible for a periodic change of the phase difference between both interfering partial beams in the interferometer. This periodic change of the phase difference corresponds to the wanted signal of interest.
Such a vortex flowmeter is known from International Patent Application Publication WO 92/01208. The demodulation of the pseudo-heterodyne signal occurs here using a phase locked loop (PLL). It has been shown to be disadvantageous in this demodulation method that the phase locked loop can set itself to higher harmonics of the pseudo-heterodyne signal and jumps between different harmonics. This strongly impacts the quality of the signal. The higher harmonics can be removed from the pseudo-heterodyne signal, but this increases the effort in terms of circuits.
Since, in the area of the tip of a sawtooth of the sawtooth signal z(t), the amplitude of the pumping current falls step-like after having achieved the maximum amplitude, this normally leads to a step-like change of the pseudo-heterodyne signal. In a possible provided amplification of the pseudo-heterodyne signal, the step-like change of the pseudo-heterodyne signal can be detected with little disturbance with an amplifier that has a high bandwidth, i.e., the step response of the amplifier leads to relatively little disturbance. An amplifier with a high bandwidth, however, normally has an undesired high energy consumption, so that the above-described technology practically cannot be used in devices, which, according to regulations, are only allowed to have a very small maximum power input, for example, two-wire devices with a current interface (4 mA-20 mA).